You can make your first withdrawal at any time! Making a withdrawal has no effect on your campaign, you will still be able to accept donations. Your withdrawal will always be for the full available balance in your WePay account.
Once you request a bank withdrawal from WePay, automatic withdrawals will stay turned on in your WePay account. These withdrawals will send you all donations received since the last withdrawal. You can change the frequency they occur to daily, weekly, or monthly by following these steps:
1) Sign in to your WePay account here: www.wepay.com
2) Click on the 'Settings' tab. Here is a picture showing where that is: http://screencast.com/t/Ir9xDYkCNt
3) Click on 'Withdrawals' on the left hand menu
4) Click 'Change'
5) Select from the drop-down menu if you would like daily, weekly or monthly withdrawals
6) Hit 'Save'
And that is it! If you request a cheque (UK only), automatic withdrawals are set to a frequency of weekly or monthly after the initial withdrawal is processed.